Evidence for calcium-mediated perception of plant symbiotic signals in aequorin-expressing Mesorhizobium loti

BACKGROUND: During the interaction between rhizobia and leguminous plants the two partners engage in a molecular conversation that leads to reciprocal recognition and ensures the beginning of a successful symbiotic integration. In host plants, intracellular Ca(2+ )changes are an integral part of the signalling mechanism. In rhizobia it is not yet known whether Ca(2+ )can act as a transducer of symbiotic signals. RESULTS: A plasmid encoding the bioluminescent Ca(2+ )probe aequorin was introduced into Mesorhizobium loti USDA 3147(T )strain to investigate whether a Ca(2+ )response is activated in rhizobia upon perception of plant root exudates. We find that M. loti cells respond to environmental and symbiotic cues through transient elevations in intracellular free Ca(2+ )concentration. Only root exudates from the homologous host Lotus japonicus induce Ca(2+ )signalling and downstream activation of nodulation genes. The extracellular Ca(2+ )chelator EGTA inhibits both transient intracellular Ca(2+ )increase and inducible nod gene expression, while not affecting the expression of other genes, either constitutively expressed or inducible. CONCLUSION: These findings indicate a newly described early event in the molecular dialogue between plants and rhizobia and highlight the use of aequorin-expressing bacterial strains as a promising novel approach for research in legume symbiosis.